生物化学甲课件史锋part ii enzyme catalysis.pptx

;Enzymes as catalysts Substrate specificity Coenzymes and Vitamines Enzyme Nomenclature;; 1.1.3 Yeast extracts were found to be able to ferment sugar to alcohol (Eduard Buchner, 1897, who won the Nobel Prize in Chemistry in 1907 for this discovery). 1.1.4 Enzymes were found to be proteins (1920s to 1930s, James Sumner on urease and catalase, “all enzymes are proteins”, John Northrop on pepsin and trypsin, both shared the 1946 Nobel Prize in Chemistry). 1.1.5 Almost every chemical reaction in a cell is catalyzed by an enzyme (thousands have been purified and studied, many more are still to be discovered!) 1.1.6 Proteins do not have the absolute monopoly on catalysis in cells. Catalytic RNA were found in the 1980s (Thomas Cech, Nobel Prize in Chemistry in 1989).;;2.2 Enzymes are highly specific both in the reaction catalyzed and in their choice of substrates (i.e., reactants). 2.2.1 An enzyme usually catalyzes a single chemical reaction or a set of closely related reactions (side reactions leading to the wasteful formation of by-products rarely occur). 2.2.2 Enzymes exhibit various degrees of specificity in accord with their physiological functions : Low specificity: some peptidases, esterases, phosphatases. Intermediate specificity: hexokinase, alcohol dehydrogenases, trypsin. Absolute or near absolute specificity: Many enzymes belong to this group, and in extreme cases, stereochemical specificity is exhibited (i.e., enantiomers are distinguished as substrates or products).;stereospecificity; 2.3.1 Some enzymes require no other chemical groups other than their amino acid residues for activity. 2.3.2 Other enzymes require additional chemical components called prosthetic groups (covalently bound) (or cofactors). 2.3.3 Prosthetic groups could be inorganic metal ions (e.g., Fe2+, Mg2+, Mn2+, Zn2+) or complex organic or metallorganic molecules called coenzymes.; 2.3.4 A complete catalytically active enzyme (including its prosthetic gr